In file advanced you made several mistakes.
Please read the file oXs_config_description.h
About NTC sensors it says:
* oXs reuses the mVolt calculated by oXs. The config must be adapted in a such a way that this mVolt is equal to the raw value returned by the ADC * 1000 (for better accuracy)
* Therefore, in section 6.1 and 6.2 ,
* USE_INTERNAL_REFERENCE must be as comment (so with // in front off)
* USE_EXTERNAL_REFERENCE must be as comment (so with // in front off)
* REFERENCE_VOLTAGE must be as comment (so with // in front off)
* RESISTOR_TO_GROUND must be set on 0 (for the index being used)
* OFFSET_VOLTAGE must (normally) be set on 0 (for the index being used)
* SCALE_VOLTAGE must be set on 204.6 (=1000 * 1023/5000) (for the index being used)
* Then you have to define some parameters in section 6.4
* FIRST_NTC_ON_VOLT_NR specify index of first voltage being used for conversion to temperature (e.g. 3 means VOLT_3) (so it is not the code of analog pin; this one is define in section 6.2)
* LAST_NTC_ON_VOLT_NR specify index of last voltage being used for conversion to temperature (e.g. 5 means VOLT_5)
* In this expample, it means that you expect to measure 3 temperatures based on NTC connected to the pins used for VOLT_3, VOLT_4 and VOLT_5
* note: if you use only one NTC, you must specify the same value for FIRST_NTC_ON_VOLT_NR and for LAST_NTC_ON_VOLT_NR
* if you do not use NTC, keep this line as comment
* SERIE_RESISTOR specify the resitance (in Ohm) connected between Arduino Vcc and NTC (and analog pin); select a value nearly equal to the resistance of NTC in the range of temperature you expect best accuracy

Here the corrections:
1) this line should be as comment (as said here above)
#define REFERENCE_VOLTAGE 3300 // set value in milliVolt; if commented, oXs will use or 1100 (if internal ref is used) or 5000 (if internal ref is not used)
becoming
//#define REFERENCE_VOLTAGE 3300 // set value in milliVolt; if commented, oXs will use or 1100 (if internal ref is used) or 5000 (if internal ref is not used)

2) There are 2 small (but important) typo fout.
You currently have:
#define FIRST_NTC_ON_VOLT_NR_1 // uncomment this line when thermistor are used; specify index of first voltage being used for conversion to temperature (e.g. 5 means VOLT_5)
#define LAST_NTC_ON_VOLT_NR_2 // specify index of last voltage being used for conversion to temperature (e.g. 6 means VOLT_6)

You can see that you have a "_" between NR and the digit. Then the compiler consider the digit as part of the name.

You should have space(s) instead. So it should be.
#define FIRST_NTC_ON_VOLT_NR 1 // uncomment this line when thermistor are used; specify index of first voltage being used for conversion to temperature (e.g. 5 means VOLT_5)
#define LAST_NTC_ON_VOLT_NR 2 // specify index of last voltage being used for conversion to temperature (e.g. 6 means VOLT_6)

3) The program has to know which arduino pin are connected to the NTC. This has to be done in line
#define PIN_VOLTAGE 0 , 8 , 8 , 8 , 8 , 8 // Fill 6 values; set to 0 up to 7 for analog pins A0 up to A7 ; set the value to 8 for the voltage(s) not to be measured.
But in you line, you only specified one pin for VOLT_1.
You have to specify too the pin used for VOLT2. If you connect it to pin A1, the line should become
#define PIN_VOLTAGE 0 , 1 , 8 , 8 , 8 , 8

Now 2 temperatures are displayed but they are not correct and are crazy.
At the SCALE_VOLTAGE value of 204.6, temperatures of ~150 ° C are displayed in the cold basement. I think 15 ° would be correct.

Changes at the SCALE_VOLTAGE shift the temperature but then the temperatures Jump from -17 to 222 ° like Crasy.

Perhaps there is a bug in oXs.
Perhaps temperature is calculated internally with 1 decimal and the Hott protocol does not accept one decimal.
In file oXs_out_hott.cpp, there is a line with
TxHottData.gamMsg.temperature1 = (voltageData->mVolt[TEMPERATURE_1_SOURCE - VOLT_1].value ) + 20 ; // Hott applies an offset of 20. A value of 20 = 0°C

Let me know if it solves the issue for temperature .
Note : use the recommended scale voltage of 204.6.
If it is ok for temperature 1, then you should change in the same way the line of code for temperature 2. It is
TxHottData.gamMsg.temperature2 = (voltageData->mVolt[TEMPERATURE_2_SOURCE - VOLT_1].value ) + 20 ; // Hott applies an offset of 20. A value of 20 = 0°C

This is strange. It looks like the voltage measured by arduino changes a lot.
Do you have have the right connection to Arduino pins.
You should have
- one pin of NTC 1 connected to Gnd
- the other pin of NTC 1 connected to arduino pin A0
- one pin of a 10k resistor connected to Arduino pin A0 (so connected to NTC1 too)
- the other pin of the 10k resistor connected to Adruino Vcc
- one pin of NTC 2 connected to Gnd
- the other pin of NTC 2 connected to arduino pin A1
- one pin of a 10k resistor connected to Arduino pin A1 (so connected to NTC2 too)
- the other pin of the 10k resistor connected to Adruino Vcc.

Please check if this is right (e.g. with a multimeter to avoid bad soldering).

If this is OK, I could explain later on, how to go deeper to debug the issue

To further debug the issue, we can first force a fix value (this just to check if the issue is in the transmission part or in the measurement part).
To do this, you can change the line
TxHottData.gamMsg.temperature1 = (voltageData->mVolt[TEMPERATURE_1_SOURCE - VOLT_1].value ) + 20 ;
by
TxHottData.gamMsg.temperature1 = 41 ;

OK, Great.
Note that a resistor of 100K (associated with a 100K NTC) is good to measure temperature around 25 degree but probably that it will be less sensitive for high temperature.
Probably that accuracy will be a little better is the resistance is about the same as the NTC resistor at the temperature that you would like to measure.

I presume you will be installing those NTC sensors on the engine's cylinder head? If so, I have similar sensors and would be interested in knowing how you did it

I did glue mine with epoxy glue to the cylinder head near the glow plug, and between 2 of the cooling fins. But when the engine gets hot, the epoxy tends to become a bit softer, and I am afraid it could come off?

Yes, I have high temperature (Red) silicone here.. how is it called? Liquid gaskets or something like that?
The problem is that it doesn't harden enough... it is just like soft rubber when dry. I don't think it would hold the sensor for long. When I was experimenting with it (and still have to finish this model), I have noticed that I really needed a good contact between the sensor and the cylinder head base, or the reading would not be accurate. The sensor would read too low.